Various processes for the manufacture of dextromethorphan are already known. One such process is described, for example, in Helv. Chim. Acta 33, 1437 (1950). According to this known process (R)- or (S)-1-1-(4-methoxybenzyl)-2-methyl-1,2,3,4,5,6,7,8-octahydro-isoquinoline is cyclized with phosphoric acid in 66% yield. The low yield in the final step is a disadvantage of this process.
Another starting material, (R)- or (S)-1-1-(4-methoxybenzyl)-2-formyl-1,2,3,4,5,6,7,8-octahydro-isoquinoline , as well as its conversion into dextromethorphan is known from Tetrahedron Letters (1987), 28, 4829 and, respectively, Japanese Patent Publication 01034964, although no details with respect to yield are present. This is not important, since this conversion is not particularly attractive. In particular, (R)- or (S)-1-1-(4-methoxybenzyl)-2-formyl-1,2,3,4,5,6,7,8-octahydro-isoquinoline has to be produced either by racemate resolution (yield.ltoreq.50%) from rac. 1-1-(4-methoxybenzyl)-1,2,3,4,5,6,7,8-octahydro-isoquinoline and subsequent N-formylation or according to the two references given above by asymmetric hydrogenation from (Z)-1-1-(4-methoxybenzylidene)-1,2,3,4,5,6,7,8-octahydro-isoquinolin-2-yl !methanone which is accessible only in a complicated manner. The development of an enantioselective hydrogenation of (Z)-1-1-(4-methoxybenzylidene)-1,2,3,4,5,6,7,8-octahydro-isoquinolin-2-yl !alkanones later opened up a simpler route for the production of (R)- or (S)-1-1-(4-methoxybenzyl)-1,2,3,4,5,6,7,8-octahydro-isoquinolin-2-yl!alka nones without racemate resolution, see e.g. U.S. Pat. No. 4,857,648.